This invention relates to the demodulation of a remote-terminal signal in the presence of interference at a receiver in a local terminal of a point-to-multipoint radio communication link wherein the interference results from a subband component of a transmitted signal at the local terminal that has been relayed back to the local receiver by a satellite transponder link. The relayed-interference signal received at the local terminal is a result of effects on the transmitted signal by the satellite uplink to the transponder, the frequency translation and amplification in the transponder, and the satellite downlink back to the local terminal. These effects modify the transmitted signal to produce the relayed-interference signal that has different amplitude, phase, frequency, and delay parameters as well as frequency dispersive parameters such as group delay. In certain prior art systems, because the transmitted signal is conventionally produced as an intermediate frequency (IF) transmit signal in the modulator in the local terminal, it is possible to transfer the IF transmit signal to the local terminal receiver and create a corresponding reference signal to the transmitted signal. The reference signal can then be appropriately modified for purposes of interference cancellation. With an acceptable level of interference cancellation, the frequencies in the transmitted signal bandwidth can be reused for satellite communication to the local receiver from other remote sites.
The invention is applied in particular to an asymmetrical point-to-multipoint satellite communication application wherein a hub terminal communicates with multiple remote terminals over a duplex satellite communications link in which a loop-back transponder sends a transmitted signal to all the terminal receivers. In this asymmetrical satellite application each of the remote terminal signals have conventionally much smaller bandwidth than that of the hub terminal transmitted signal. The transponder is designated loop-back or alternatively “bent-pipe” as its operation is limited to bandpass filtering, frequency translation, and amplification and does not include demodulation and remodulation. Conventionally such a satellite communication link can only transmit and receive signals in one direction for a single access use. For example in a frequency-division multiple access system, a separate bandwidth allocation for the hub terminal and a separate bandwidth allocation for a remote terminal would be necessary for communication in both directions. However, the interfering signal at the transmitter terminal could be generated at the same terminal receiver to cancel its “self” interference thus providing reuse of the frequencies in the hub terminal bandwidth allocation for return link communication between the remote terminal and the hub terminal.
Conventionally in this asymmetrical application a hub terminal employs a larger diameter antenna, with gain GH, than a remote terminal with an antenna of gain GR, such that GH>>GR. Hub terminal antennas may be as large as 18 meters and remote terminal antennas may be as small as 1.2 meters producing an antenna gain differential of approximately 24 dB. For both signal directions the received bit energy is proportional to the antenna gain product GHGR and the transmitted energy per bit. In data transmission with a fixed modulation type the bit error probability is proportional to the received bit energy so since the antenna gain product is the same in both directions the transmitted energy per bit can be about the same for the hub-to-remote direction as for the remote-to hub direction. However, because the transponder relays the transmitted signal back to the same terminal, the relayed interference signal has received bit energy proportional to the local terminal antenna gain squared. Consequently, the ratio of signal to interference spectral density at the remote terminal is GH/GR whereas that ratio at the hub terminal is equal to the reciprocal GR/GH. Since this asymmetric satellite application has GH>>GR, there is generally no interference problem for reused frequencies at the remote terminal and a big interference problem for reused frequencies at the hub terminal. In satellite systems, bit-error rate performance goals are typically within 0.3 to 0.5 dB of theoretical limits. The cancelled relayed-interference signal is approximately complex Gaussian distributed so that its power adds to the channel noise at the receiver. If 0.4 dB is allocated for performance degradation due to a residual relayed-interference signal, the cancellation must push the relayed-interference signal approximately 10 dB below the noise. Additionally for the hub terminal signal, there is generally a larger fade margin requirement and a larger energy per bit requirement due to a higher order modulation. These two factors can increase the relayed interference signal by an additional 6 dB relative to the desired remote terminal signal. Thus, for a signal-to-noise ratio of 5 dB for the desired remote terminal signal, the required cancellation would be equal to 24+6+5+10=45 dB.
In addition to 45 dB of required cancellation, there is a very large dynamic range required for baseband digitizing the received signal at the hub terminal so as to successfully capture the weak desired remote terminal signal buried in the strong relayed-interference signal. The hub terminal conventionally sends separate signals with a single carrier in a time-division multiplexed (TDM) format to each of the remote terminals. The remote terminal sends back information to the hub terminal on a single carrier as part of a frequency-division multiplexed (FDM) format or a combination of FDM and TDM formats. Accordingly, the symbol rate for the hub terminal, RH, is much larger than a remote terminal symbol rate, RR. Typical values for symbol rates would be 10 MHz for the hub terminal signal and 50 kHz for the remote terminal signal, a dB difference of 23 dB. The ratio of signal to interference power at the hub terminal is the spectral density ratio difference with a maximum of about 30 dB, as discussed above, plus 10 LOG (RR/RH). Thus, for conventional systems the interference power can be about 30+23=53 dB larger than the remote received signal power at the hub terminal. For successful demodulation of the remote-terminal signal the dynamic range at the hub terminal receiver must be significantly larger than this value.
The frequency converter in the satellite upconverter for the transmitted signal path and frequency converter in the satellite downconverter in the received signal path both add phase noise variations that when combined are on the order of 30 to 40 dB smaller than the hub terminal signal. Since the maximum required cancellation is about 45 dB, it is clear that these phase noise variations need to be compensated if cancellation goals are to be attained.
Prior art systems have been developed to provide multiple-access reuse in full-duplex satellite communication systems operation with a loop-back transponder. These systems use signals associated with the terminal transmitter to produce a reference signal for purposes of cancellation of the “self” interference at the receiver. U.S. Pat. No. 5,596,439, “Self-Interference for Two-Party Relayed Communication”, Mark D. Dankberg, et al., discloses an open-loop technique consisting of measurement techniques followed by interference reduction based on measured link parameters that are applied to the reference signal. Errors in parameter measurement can significantly degrade the subsequent interference reduction. It is generally recognized that an adaptive closed-loop system can be more effective in cancellation systems with variable link parameters.
U.S. Pat. No. 6,859,641 B2 “Adaptive Canceller for Frequency Reuse Systems”, Glen D. Collins, et al. (“'641 patent”), and U.S. Pat. No. 7,228,104 B2 “Adaptive Canceller for Frequency Reuse Systems”, Glen D. Collins, et al., (“'104 patent”), disclose an adaptive cancellation system that converts a sample of the IF transmitted signal to digital form and converts the IF received signal containing the relayed interference to digital form. Frequency, phase, gain, and delay parameters of the sample of the transmitted signal are adjusted to produce a compensating signal that is added to the digital form received signal to produce a signal of interest. The signal of interest can be converted back to an intermediate frequency for interface with a down-stream demodulator.
U.S. Pat. No. 7,522,877 B1 “Noise Reduction System and Method Thereof”, Abel Avellan, et al., (“'877 patent”), discloses an interference-reduction system for the hub terminal in the asymmetrical satellite communication configuration described above. The interference-reduction system digitizes and converts to baseband the hub terminal IF transmit signal and transfers the bits in the baseband digital signal to a buffer in the hub receiver to produce a replica of the hub transmitted signal. The replica is then scaled, delayed and distorted to reduce the transponder-relayed hub interference signal in the aggregate received signal that also contains multiple remote terminal signals. A finite-impulse-response (FIR) filter forms the basis for an adaptive delay equalizer that uses a set of real weights that adapt to compensate for fractional-sample delay errors. Since the interference reduction is over the hub signal bandwidth rather than a single remote terminal signal subband, the effects of nonlinearities in the hub transmitter can critically limit interference reduction. Accordingly, the '877 patent discloses the generation of AM-Normgain and AM-PM correction arrays that are used for the distortion modification of the hub transmitted signal replica.
The Collins '641 and '104 patents and the Avellan '877 patent also disclose transfer from the hub transmitter to the hub receiver of the IF transmit signal. By using this signal, problems associated with variations in the transmitted amplitude and data clock are avoided. However transferring the IF transmit signal requires a significantly larger bit transfer rate than transferring the digital data signal that produces the IF transmit signal resulting in a higher cost system. Further, the Collins '641 and '104 patents and the Avellan '877 patent do not address the large dynamic range required and the limitation of cancellation caused by the presence of frequency-converter phase noise as discussed above.
It is further recognized that a critical problem to be solved in these interference reduction systems is precise tracking of frequency and time-delay error. The frequency errors include local oscillator variations in the satellite upconverter for the transmitted signal path, local oscillator variations in the satellite downconverter in the received signal path, and Doppler error due to satellite motion. Time-delay errors result from variation in path delay between a terminal and the satellite due to satellite motion. Intelsat satellites for example, have a maximum delay variation of about 0.43 milliseconds peak to peak over a 24-hour period. The moon's gravitational pull also interacts with the satellite orbit. For Intelsat satellites the maximum rate change is about 15.4 nanoseconds/second. Other satellite operators may have higher numbers. These frequency and time-delay errors are present in both the interfering transmit signal component and desired remote-terminal signal component in the received signal. Because of the different paths to and from the satellite these errors are not the same for both components.
Accordingly, there is a need at a hub terminal in these asymmetrical satellite link applications for demodulation of a remote-terminal signal that includes cancellation of a component of relayed-interference signal in the remote-terminal signal subband in the face of frequency-converter phase noise and multiple varying link parameters that can significantly degrade the cancellation factor.